1. Field of the Invention
The subject invention generally relates to a method of acquiring image data with a magnetic resonance imaging (MRI) system. More particularly, the invention relates to a method of acquiring MRI image data using a sequence of radio frequency (RF) pulses.
2. Description of the Related Art
In magnetic resonance imaging (MRI), a conventional MRI system includes a field magnet that is energized to produce a substantially homogeneous static magnetic (B0) field through a bore in which an object, typically a human body, is placed. The object includes a plurality of atomic nuclei, typically belonging to hydrogen atoms, within organic matter such as tissue, bones, etc. Each of the atomic nuclei exhibits an intrinsic angular momentum and spins about an axis. The magnetic moment of each of the atomic nuclei is known as a spin. The B0 field induces the spins to align with a longitudinal axis defined by the B0 field. The conventional MRI system includes an RF transmitter that temporarily applies an RF pulse to the object to rotate the spins away from the longitudinal axis. Thereafter, the spins precess (rotate) about a plane that is transverse to the longitudinal axis. Gradually, the spins realign to the longitudinal axis as a result of longitudinal relaxation (T1) and transverse relaxation (T2). In so doing, the spins induce a detectable nuclear magnetic resonance (NMR) signal. The conventional MRI system includes gradient coils that generate and temporarily apply magnetic field gradients to the object for determining the spatial location of the spins. The conventional MRI system includes an RF receiver for receiving the NMR signal induced by the spins and a computer for processing the NMR signals to form part of an image corresponding to a scanned region of the object.
The sequence of RF pulses and magnetic field gradients may be repeated every “TR” milliseconds, where TR is an abbreviation for “sequence repetition time.” If TR is relatively shorter in duration than the transverse relaxation time T2, the spins will not have sufficient time to realign to the longitudinal axis prior to the application of each RF pulse. As a result, the spins generally approach a dynamic steady-state condition that is a function of tissue relaxation parameters (T1, T2) and imaging sequence parameters, such as flip angle and TR.
One common method of steady-state MRI imaging is balanced steady-state free precession (bSSFP) imaging. Balanced SSFP is also known as balanced fast field echo, FIESTA, and TrueFISP imaging. Balanced SSFP utilizes RF pulses combined with balanced magnetic field gradients, i.e., gradients that act on spins between consecutive RF pulses and preserve the phase of the spins existing before application of the gradient.
Unfortunately, bSSFP imaging suffers from considerable signal loss and off-resonance artifact as a consequence of B0 inhomogeneity. Specifically, a significant loss of SNR and useful image contrast occurs in regions where the B0 frequency offset is an odd integer multiple of one-half of the inverse of TR. Consequently, dark bands may be present in the image, which is undesirable and limits the application of bSSFP imaging. Another limitation of bSSFP imaging is the difficulty of inserting “magnetization preparation” RF pulses that can, for example, suppress fat or alter image contrast in some desirable way.
Accordingly, there remains an opportunity to provide an MRI imaging system and method that overcomes the aforementioned problems.